Low legume share in France

France has a low share of protein crops, such as peas and lentils, due to harvests that are not always profitable. These legumes, however, can reduce the need for nitrogen fertilizers if their role in soil health is properly considered.

Legume use in European agriculture

Less than 5% of large-scale crop areas in France are legumes, compared to 15-20% in other continents. This low proportion is an exception in Europe, according to Anne Schneider, a specialist in grain legumes at Terres Inovia. It makes French agriculture more dependent on mineral fertilizers for nitrogen.

Nitrogen fixation by legumes

Legumes can fix nitrogen from the air, with the amount varying by species. For example, vetch and lupin can fix up to 90% of their nitrogen needs, while peas and lentils fix between 55-75%. However, legumes tend to use soil nitrogen when it is available, though this varies by species.

Nitrogen return after legumes

The amount of nitrogen left in the soil by legumes is not easily determined by the nitrogen fixation rate alone. Vetch is considered the best protein crop for nitrogen return, followed by peas. Soy is less effective because it exports a lot of nitrogen in its grain and has a weaker root system.

Reducing fertilizer use

Studies suggest that after a legume crop, nitrogen fertilizer can be reduced by up to 60 units for a cereal crop like wheat, depending on local conditions. This is based on data showing an average yield increase of 7.5 quintals per hectare after peas.

Legume impact on rapeseed

Rapeseed uses nitrogen early in the season, as early as autumn. After a legume, the nitrogen dose can be reduced by up to 50 units while maintaining yield. This is less risky than reducing nitrogen after a cereal crop, according to Anne Schneider.

Soy’s role in nitrogen management

Soy, harvested later than other legumes, is more beneficial for summer crops like corn. It can reduce nitrogen use by 30 to 40 units while increasing corn yield by 10%, due to a better match between nitrogen availability and crop needs.

Biomass and nitrogen capture

The amount of nitrogen a legume returns to the soil depends on its biomass at harvest. To capture nitrogen effectively, it is important to use intermediate crops or species mixtures, especially crucifers, which are efficient in nitrogen capture.

Optimal legume cultivation

To ensure a good nitrogen return, legumes need proper soil and sowing conditions, including a well-prepared, non-compacted soil and adequate phosphorus and pH levels. This helps in developing a strong root system and nodules for nitrogen fixation.